1. Field of the Invention
The present invention relates to, in a light emitting device comprising an LED chip, an improvement in a sealing layer that encloses the LED chip.
2. Description of the Related Art
An LED chip to be mounted on a light emitting device is sealed with a sealing layer. The sealing layer is required to have not only transparency of a light emitted from the LED chip but also various characteristics such as light resistance, heat resistance, chemical resistance, and insulating properties.
In a light emitting device that is required to exhibit a particularly high output, for a forming material of the sealing layer, a high light resistance is required as light intensity of the LED chip is increased and a high heat resistance is required as heat generation in the LED chip increases.
As such a forming material of the sealing layer, a silicone composition has been proposed. For example, refer to JP-A-2005-327777, JP-A-2009-173789 and JP-A-2010-168443.
With an increase in the output of the light emitting device, an enlargement in physical dimensions of the light emitting device itself, so-called an increase in size (upsizing) is sometimes required.
As one type of the light emitting device, there is one having a configuration where an LED chip is mounted on a substrate, the LED chip is surrounded with a cup-shaped case part, the inside of the case part is filled with a silicone composition, and thus the LED chip is enclosed with a sealing layer composed of the silicone composition. In the light emitting device having such a configuration, the diameter of the case part is increased with an increase in the size thereof. On the other hand, the thickness (height) of the case part is sometimes restricted and, as a result, the sealing layer that encloses the LED chip is flattened. When repeated thermal histories are applied to such a flat sealing layer, it has been found that, even when the forming material is a silicone composition intrinsically having rubbery properties, there is a concern that cracks are generated therein.
On the other hand, in a versatile light emitting device in which the diameter of the case part is about 2 mm, the phenomenon of crack generation in the sealing layer does not occur and there has not been performed investigations for preventing the crack generation. In other words, any parameters that may cause crack generation in the sealing layer have not been investigated.